It is a general tendency of the recent LCD market that a display capacity (a number of pixels) is increased in step with enlarging a screen size. To obtain a uniform display quality all over a large screen, how to process an orientation is a critical factor. An LCD has been oriented through rubbing a surface of polyimide orientation film with a rubbing buff. This film is made from thermesetting polyamic acid, and the rubbing buff is made of a cloth on which short fibers are transplanted. This orientation method requires simple process, a little time and reasonable cost.
This method, however, entails scrapes on the orientation film due to mechanical contacts between the rubbing-buff-tips and surface of the film. Strength of these mechanical contacts and a degree of scraping are greater at level differences on a substrate than other places, and particularly greater at edge portions thereon. The scrapes remaining on the surface sometimes produce some influence.
The striped mottles produced by an orientation and appearing on a color filter surface of a substrate, in particular, have not been eliminated although a rubbing condition has been changed. One example of the condition changes is that the buffing face is pressed vertically to the orientation film surface so that rubbing density can increase.
Striped mottles appearing on a moving direction can be illustrated in FIG. 7 in which a thin disc represents a rubbing roller because the thin disc contacts a substrate surface at only one point for making a description simple. Thin disc 21 contacts substrate 7 atone point and rolls on shaft 19b of rubbing roller 19 in the direction indicated by arrow mark "b". Solid line 20 showing a moving direction (arrow mark "a") of glass substrate 7 and another solid line 20a showing a rotary direction projected on the surface of glass substrate produce an angle .theta.. A prior art disclosed in the Japanese Patent Application Unexamined Publication No. H02-22624 teaches that the angle .theta. is adjusted within the range of .+-.1.degree.-45.degree. so that the striped mottles can be eliminated.
The orientation process discussed above, however, is easily affected by the level differences on the surface of substrate because the thin disc contacts the surface at only one point. In the present market trend of increasing a display capacity, in particular, a transparent-conductive-film (ITO=indium-tin-oxide) is thickened in order to lower a resistance of ITO for overcoming cross-talk and variation of threshold values due to attenuation by an electrode. When an electrode pattern is formed on such a thickened ITO, grooves having level differences of several thousand angstroms appear linearly aligned. In the prior art where the thin disc having only one contact point with the film, the conventional rubbing buff of which fiber direction is the same as a rolling direction of the rubbing roller produces striped mottles due to these grooves.
The grooves produce influence at every rotation of the rubbing roller, and the influence is effected in series because of continuous rolling, whereby the striped mottles can be recognized while the LCD is operated. The mottles thus produced on LCD 24 are shown in FIGS. 11. There are various types of mottles relative to places and widths such as "a1" parallel with a moving direction of the substrate, or wider (belt-like) mottles "a2".